The death toll in Syria during eight months of protests has now risen to "much more than 4,000", the UN revealed on Thursday as Britain, the EU and Arab states all ratcheted up the pressure on the embattled regime of President Bashar al-Assad.

Navi Pillay, the UN human rights commissioner, said Syria could now be considered to be in a state of civil war. On Monday a UN investigation accused the Damascus government of committing crimes against humanity and said 256 children had been killed since unrest began. On 9 November the UN said the death toll was 3,500.

"We are placing the figure at 4,000, but really the reliable information coming to us is that it is much more than that," Pillay told a news conference in Geneva.

The updated death toll was issued as Britain accused Iran of aiding repression in Syria, as part of its strategy of soliciting wider support for tougher sanctions against both governments. Syria says it is fighting armed terrorists supported by a conspiracy of its Arab and western enemies.

William Hague, the foreign secretary, in a hardening of the UK position, said: "There is a link between what is happening in Syria and what is happening in Iran."

Hague said before meeting EU colleagues in Brussels: "The Iranian government has given assistance to the Assad regime in trying to control and suppress protests."

Arab and western governments have privately pointed to evidence that Iran is providing financial and logistical assistance and active security advice to the Assad regime. According to one senior Arab diplomatic source, there are 300-400 Iranian revolutionary guard personnel in Syria.

But it is unusual for a foreign minister to make such an accusation in public – reflecting an increasingly tough stance towards Syria and Iran in the wake of this week's Tehran embassy crisis.

The EU named 12 more Syrian individuals and 11 entities as being subject to travel bans or asset freezes. Syria immediately retaliated by announcing it was suspending its membership of the EU's Union for the Mediterranean – a partnership grouping.

The Arab League, represented in Brussels by its secretary general, Nabil al-Arabi, also named 17 senior Syrians it said would be denied entry to Arab countries. These include Assad's brother and republican guard commander Maher, the architect of the security crackdown; his billionaire cousin Rami Makhlouf, and the ministers of defence and interior.

Arab flights to Syria will stop from mid-December. But wheat, medicine, gas and electricity are excluded from the embargo, which is supported by all 22 Arab League members except Iraq and Lebanon. Syria has been suspended from the body.

Hague's comments and the latest sanctions came on a day that saw at least 17 people reported killed, mostly in the Hama area, according to the Syrian Revolution General Commission.

In recent weeks the situation has been complicated by increasingly frequent armed attacks against regime targets by the Free Syrian Army (FSA), which is made up of defectors from the armed forces.

In another development, the Syrian National Council (SNC), the biggest anti-Assad grouping, announced that it had reached agreement with the FSA over the issue of attacks against government forces at a meeting held in southern Turkey on Monday. Turkish media reported that the FSA would now resort to "armed resistance" only for "defensive" reasons.

Khaled Khoja, an SNC member, said: "The council recognised the Free Syrian Army as a reality, while the army recognised the council as the political representative [of the opposition]."

Opposition demonstrations across Syria on Friday are to be held under the slogan of demanding the creation of a border "buffer zone" to protect civilians. Kuwait, meanwhile, called on its citizens to leave Syria, following similar calls by several other Gulf states in recent weeks.